An ultrasound diagnostic apparatus to produce an ultrasound image transmits an ultrasound from an ultrasound probe to a subject, receives an echo from the subject by the ultrasound probe, reconstructs an ultrasound image based on signals received from the ultrasound probe and displays the reconstructed image.
As one such ultrasound diagnostic apparatus, there has been known an apparatus that creates an elastic image revealing properties of living tissues of a subject, such as strains and hardness. For example, the ultrasound diagnostic apparatus acquires time-sequenced images of a living tissue as the subject is applied a pressure, measures a displacement of the tissue by taking a cross-correlation among the acquired time-sequenced images and determines elasticity data (e.g., strain and elasticity modulus) based on the measured displacement (e.g., Patent document 1). In measuring the displacement of a living tissue, one method of applying a pressure to the subject may involve, for example, manually pushing the ultrasound probe against the subject with a force.
A point spread function in ultrasound imaging is normally short in an ultrasound propagation direction and spread in a direction perpendicular to the propagation direction (the latter direction is hereinafter referred to as a lateral direction). So, a local displacement measurement is done only in the propagation direction and not in the lateral direction. In practice, there is a case where a direction in which a living tissue is actually displaced when a pressure is applied to the subject (hereinafter referred to as a tissue displacement direction) may not be parallel to an elasticity calculation direction in which the displacement of the living tissue is measured (hereinafter referred to as a displacement estimation direction). That is, the displacement estimation direction is fixedly set in a direction perpendicular to an ultrasound transmission/reception surface, whereas the tissue displacement direction changes unstably depending on the direction in which the tissue is pressurized or the shape of a pressing surface. To adjust with this situation, a method is available which matches the displacement estimation direction with the tissue displacement direction (e.g., Patent document 2).    Patent document 1: JP-A-2004-57653    Patent document 2: WO 2006/073088